Building a “hum-free” audio is not easy till you know what you are doing. It is not as easy as connecting wire into where it should be connected. Especially for grounds, if you connected it into wrong point. Noise usually come from ground loops or connecting a very sensitive signal ground line to a high current ground path. In most cases a star connection is very effective in eliminating the noise.
There are lots of bluetooth speaker on the market today. But if you have an old mini component or a sound system that is still working and sounds good, why won’t you just upgrade it and put a bluetooth functionality? There are lots of audio bluetooth receiver on the market today.